Another option is to create a sign-up form via the Sign-up Tools and in the html, you can find the list number in it's hidden field:
<input data-id="list:input" name="list" value="1" type="hidden">
Unbelievable that this number isn't shown in the UI!
DaveBerard, your claim that, "The List ID number is only useful for developers who are using our API..." is not really accurate. I'm a developer and need to remove contacts from a list through the API, but I need to be specific about which list they are removed from based on activity they've performed on my website. The only way I would have of identifying a contact's "correct" list is if the ConstantContact account owner feeds the list ID it into my system to begin with.
I've performed this exact same style of integration with MailChimp and Real Magnet, both of whom permit a list to be accurately identified. I wish Constant Contact did also. Until then, it appears I'll not be able to integrate my system with Constant Contact.
There are a few other options listed in this thread for how to get the list ID without using the API; however from what you described it sounds like you would be able to use our API to get the list IDs needed for your integration. If you would like to provide details about your process I'd be happy to see if there is some way to come up with a solution. Please send us an email at firstname.lastname@example.org.
To be specific about ReginaB41's steps on how to get the ID without having to build some kind of utility, here are some details. I use Firefox's Developer Tools (F12).
I can't imagine a reason NOT to add the list ID to the UI, especially considering how trivial it would be for you to do it. Here's a good reason why it should be there: Let's say I'm a developer, and I'm taking over a project where your API has been implemented to add users to a list when the user submits a form. Now the client has a new list and wants me to have users go to that list instead. I can clearly see where to update the list ID in the existing code, but what is the list ID of this new list???
You see, just because a developer is working with your API doesn't mean that developer has time to get to know your API, or has any good reason to. In this scenario, having the list ID available in the UI would save a developer time, and keep them from thinking bad thoughts about Constant Contact.
You can simply add the ID as a data-id on the html element containing the list name, just as you've added your internal ID (I guess that's what that is) to the tr element. Developers will find that quickly. You don't have to change the look and feel of your UI.
This is a very good use case and I will pass this along.